In the thermomechanical pulping (TMP) process, the wood material is introduced in form of chips together with water between the rotating discs of a refiner. Electrical energy is added and a large portion of the water heated is converted into steam.
The woodchips to be refined undergo a three step treatment before entering the refiner. In the first step, the woodchips are presteamed at atmospheric pressure by steam from a preheater. After being heated to about 70.degree.-90.degree. C., the chips are placed into a chip washer where debris like sand is removed. The washed chips are then fed by a screw feeder under high pressure into the preheater. The turpentine condensed on the chip surface in the presteamer and pitch are squeezed out together with the excess water from the chip washer. The steam used in the preheater is steam flowing back from the refiner. Although only about 20% of the steam generated in the refiner flows back to the preheater, this backflow steam contains generally over about 80% of the total turpentine content. Turpentine is therefore normally not recovered but is removed together with the waste water.
U.S. Pat. No. 3,816,239 proposes a method for recovering terpenes from a wet, gaseous steam containing terpene vapor and water vapor by condensing water from the steam by indirect heat exchange to produce an aqueous liquid terpene-lean condensate phase while leaving a first terpene-rich vapor phase. The aqueous terpene-lean condensate is separated from said vapor phase, subcooled and used as the indirect heat exchange medium at boiling for condensing additional terpene-lean condensate from the wet, gaseous stream. During this indirect heat exchange, a portion of the terpene-lean condensate is flash vaporized and these distillate vapors are combined with the first terpene-rich vapor phase for recovery of terpenes therefrom by condensation at a temperature below the dew point temperature of the terpene component. This patent does not, however, propose the recovery of the heat from the gaseous stream for the purpose of heating wood chips.
It is an object of the present invention to recover turpentine which is boiled off from the wood together with water vapor during the woodchips refining process.
In addition to the recover of turpentine contained in the preheater vent and refiner backflow steam, it is a characteristic feature of the present invention that heat is recovered from the refiner backflow steam by condensing a major portion of the vent vapor. If no boiler feed water is used as a medium for generating steam for presteaming the woodchips, the condensate of the vent vapor in the evaporator condenser reboiler is reboiled so as to produce steam for the preheating and presteaming the woodchips to be refined. On the other hand, if the steam for presteaming the woodchips is generated from boiler feed water, clean steam will be produced and it is not necessary to first condense and then reboil the condensate so as to produce steam for the presteamer.